(1) Field of the Invention
The present invention relates to an electrodeposition coating film-forming composition capable of forming a coating film with varistor property and to a an electrodeposition coating method using said composition. In detail, it relates to an electrodeposition coating method capable of forming a coating film which imparts high throwing property, corrosion resistance, and edge cover property, by electrodeposition coating an electrodeposition coating film-forming coating composition containing a powder of an electrically semiconductive substance as a first coating layer, said coating film having varistor property and being smooth and excellent in corrosion resistance, on which a second coating layer can be further provided by electrodeposition coating.
(2) Description of Related Art
Hitherto, it has been considered to be difficult in principle to once provide deposits on a metal article to be coated by the electrodeposition coating method and after baking, further provide a second coating layer on the resulting coating film by electrodeposition coating because the first coating layer becomes electrically insulative.
However, in Japanese Patent Publication No. 24566/74, there is found an example of such two-layer electrodeposition by electrodeposition coating. The method of this Japanese Patent Publication No. 24566/74 is concerned with a method in which using black iron oxide as an essential component and an anionic electrodeposition paint composition as a vehicle, an article to be coated is subjected to electrodeposition coating and baked to form an electrically conductive coating film with marked properties regarding adhesive strength, electrical conductivity, surface characteristics, and corrosion resistance, and a second coating layer is further provided thereon by electrodeposition coating.
Furthermore, in Japanese Patent Kokai (Laid-Open) No. 51958/84, in order to improve the quality of the method of the aforesaid Japanese Patent Publication No. 24566/74, a coating film as a first coating layer is formed from a combination of a cationic electrodeposition paint composition with, as an electrically conductive substance, finely divided carbon, i.e., carbon black and/or graphite, whereby uniformity of the resulting coating film and high throwing property are attained, and in order to further improve the corrosion resistance, lead compounds such as, for example, lead silicate, lead chromate, lead acetate, or lead lactate are added.
As seen in the aforegoing examples, in order to form an electrically conductive coating film by the electrodeposition coating method, the use of an electrically conductive substance is necessary. Up to date, black iron oxide, carbon black, graphite, and metal powders (e.g., zinc, copper) have been known as the electrically conductive substance.
However, in the case of using black iron oxide as an electrically conductive substance in an electrodeposition coating composition as in Japanese Patent Publication No. 24566/74, there is involved a defect that since black iron oxide has a high specific gravity and is used in the state of an aqueous solution in which the electrodeposition paint is diluted, the precipitability is very vigorous so that in the case that an article to be coated has a complicated shape, a uniform coating film in the respective portions is not formed.
Furthermore, in the case that carbon black and/or graphite which is finely divided carbon is used as an electrically conductive substance as in Japanese Patent Kokai (Laid-Open) No. 51958/84, since the electrodeposition coating is carried out by using a cationic electrodeposition paint composition but not an anionic electrodeposition paint composition, the corrosion resistance of a formed coating film is improved. However, since carbon black and/or graphite does not basically have rust resistivity, the electrodeposition paint is inferior to usual cationic electrodeposition paints free from carbon black and/or graphite in terms of the rust resistivity, i.e., corrosion resistance. Moreover, though it is described that in order to improve this corrosion resistance, it is preferable to add a lead compound, there is involved a defect that in the case that an electrically conductive electrodeposition paint contains such a substance(s) in an amount necessary for attaining the generally required corrosion resistance in cationic electrodeposition paints, the dispersion is difficult, and a uniform electrically conductive coating film is hardly obtained.